The invention relates generally to the field of circuit elements which are produced using thick-film technology. More particularly, the invention relates to an improved switch element having good wear characteristics that can be cheaply and reliably produced using known equipment and materials.
Switching and encoding electronic components are prevalent in many industries and products. Sliding electrical contacts interfacing with robust metal terminals have been sufficient for simple switching applications and high electrical loads. However, with the increasing emphasis on electronics in product design, and the concomitant proliferation of complex switching patterns and relatively low electrical loads, the prior sliding contact technology has become ineffective. The increasing technological demands have given rise to printed circuit elements involving etched or deposited conductor patterns on a non-conductive substrate.
Circuit elements comprising pyrolytically deposited films of electrically conductive material on a ceramic substrate are well known in the art. For example, the patent to Wahlers et al., U.S. Pat. No. 4,397,915, discloses a vitreous enamel resistor material which is applied to a ceramic substrate and fired to produce an electrical resistive element. Likewise, thick-film circuit technology is equally well known, albeit of more recent origin. A variety of electronic circuit elements have been produced using thick-film circuit technology, such as resistors, capacitors, and switches.
More recent advancements in thick-film technology have been in the development of thick-film cermet inks which are applied to a substrate in a specific circuit pattern. The cermet inks typically comprise a metal conductive component within a glass or ceramic matrix. Typically the metals are noble metals such as ruthenium, platinum, gold, rhodium, palladium and silver, as well as oxides of the noble metals.
The use of thick-film cermet inks in the production of resistive elements is thought to minimize contact resistance while maximizing durability, stability, and tarnish resistivity. For example, the patent to Bosze et al., U.S. Pat. No. 4,824,694, described a resistive element employing a thick-film cermet ink applied to an insulative substrate. The Bosze cermet resistive element attempts to address the problem of increasing tarnish resistance and reducing surface resistivity of the circuit element at the point of contact with a wiper element. The Bosze element accomplishes this function by the use of discrete spaced-apart islands of predominantly conductive material applied to the cermet resistive layer which reduces the contact area against the wiper while maintaining adequate electrical resistance.
The patent to Crook et al., U.S. Pat. No. 4,771,263, represents yet another approach to the production of a variable resistance element which is intended to improve the life of the switch components, namely the variable resistor and the contact wiper. The Crook et al. resistance strip includes a ceramic substrate upon which a high temperature glass layer is applied. A thick-film resistive past is then applied to the glass substrate to act as the principal resistance strip. A second thick-film ink is then applied over the first ink that acquires a glass-like sheen after firing. The object of the Crook et al. resistance strip is that the resistive elements are applied to a smooth glass base, rather than to a ceramic base, thereby adopting the surface texture of the high-temperature glass layer.
While the foregoing technology has been adequate in the design of thick-film resistors and variable resistance elements, switch elements present a different problem that is not addressed by this prior art technology. More particularly, switch elements typically comprise a conductive strip surrounded by insulating material that must be accessible to a resistive wiper element. As the wiper passes over the strip the switch is triggered. However, in the thick-film switch elements of the prior art, the conductive strip is exposed above the surface of the insulating portion of the element. Thus, as the wiper element passes repeatedly over the resistive strip, the wiper and the resistive strip are gradually worn.
Some switch elements have been produced in which an epoxy filler is applied between etched precious metal conductor strips. The epoxy filler, or other insulating material, is applied to eliminate step height problems between the conductor and the base substrate. Although these types of switch elements have superior wear life and high corrosion resistance, their manufacture is typically too costly to be used in many applications and products.
Consequently, there remains a need in the art for a thick-film electrical switch element that has good contact life, smooth mechanical operation, and satisfactory electrical performance. It is also desirable that this switch element be capable of inexpensive production, preferably using presently available equipment and materials.